New hints on nutrients dynamics and their impact on carbon sequestration in the Mediterranean Sea
Ibello, Valeria (2009) New hints on nutrients dynamics and their impact on carbon sequestration in the Mediterranean Sea. [Tesi di dottorato] (Inedito)
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In the World Ocean nitrogen and phosphorus cycles are deeply linked to the carbon cycle. As nitrate and phosphate play an important role in controlling biological production, the intensity of atmospheric carbon sequestration is strongly influenced by nutrient bioavailability. This thesis aims to contribute to the understanding of the ability of the Mediterranean Sea in absorbing C and N from the atmosphere. The Mediterranean Sea presents several biogeochemical anomalies compared to the global ocean. The high nitrate/phosphate ratios in subsurface waters and the low 15N/14N ratios in particulate and dissolved nitrogen have suggested a significant occurrence of nitrogen fixation (N2 fixation) or an important impact of nitrogen atmospheric deposition. This study presents, for the first time, a basin-wide overview of direct measurements of N2 fixation, with values in the North Atlantic for comparison. Very low N2 fixation rates (0.0520.031 nmols N l-1d-1) were observed in all sub-regions of the Mediterranean, in contrast to the higher values measured in the North Atlantic (0.3000.115 nmols N l-1d-1). Higher phosphorus (inorganic or organic) concentrations were not associated with higher N2 fixation rates. An isotopic evaluation of the nitrogen sources based on isotopic signature of deep nitrate, N atmospheric deposition and Atlantic inflow nitrate, provides a rough estimate of the amount of N atmospheric deposition. The range varies between 22 and 60% of the total nitrate entering the Mediterranean Sea. Large amount of N deposition from the atmosphere can explain both P-limitation and anomalous N/P ratio in deep layers. The external inputs of N, determining the excess of N in the marine system could ultimately fuel new production and enhance Mediterranean’s ability in carbon sequestration.
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